1. Field of the Invention
The present invention relates to an image forming apparatus in which a magnetic latent image, an electrostatic latent image, or the like, is formed on, e.g., an image forming body, is developed with a toner; the obtained toner image is transferred onto a conveyed transfer material, and the transferred toner image is fixed to form a visible image. More particularly, the present invention relates to improvement of feeding and conveying mechanisms of the transfer material.
2. Description of the Prior Art
In a conventional image forming apparatus, in order to transfer a toner image formed on an image forming body at a proper timing, a transfer material (or transfer sheet) conveyed from a sheet feed cassette is brought into contact with and stopped at registration rollers in advance, and the registration rollers are driven in synchronism with a scan start operation of an optical system for reading original information, thereby conveying the transfer sheet to a transfer region.
Along with the advance of recent copying techniques, it is desired to more accurately reproduce an original image on a transfer sheet. For this purpose, the following problems are posed for the prior art in which the transfer sheet is conveyed to the transfer region in synchronism with an image formation timing under the control of the registration rollers.
(1) The position of the leading end of the transfer sheet stopped at the registration rollers is indefinite, and an offset of timing occurs more or less in every transfer.
(2) When the registration rollers are driven in response to a scan start signal to convey the transfer sheet, since the rollers often do not satisfactorily catch the transfer sheet, the transfer sheet slips. As a result, a conveying operation is delayed or the transfer sheet is bent.
For example, Japanese Patent Laid-Open (Kokai) No. 62-208065 has been proposed. In this prior art, a transfer sheet fed from a sheet feed cassette is conveyed by registration rollers to a first position in front of the registration rollers. At the first position, the conveying operation of the transfer sheet is decelerated to, e.g., a low speed 1/4 of that in a sheet feeding/conveying mode by decelerating a drive motor. Subsequently, the transfer sheet is conveyed to and stopped at a second position. Thereafter, the transfer sheet is conveyed from the second position as a reference point to a transfer region of an image forming apparatus in synchronism with an image formation timing. A toner image formed on an image forming body is transferred to the conveyed transfer sheet. The detailed content will be briefly described with reference to FIGS. 1 and 2. More specifically, a transfer sheet P stacked on a cassette 6 is fed and conveyed by a transfer sheet pickup roller 7, and is then caught and conveyed by registration rollers 8. The transfer sheet P then reaches a first position A, and its leading end is detected by a first sensor 11a. The detection signal from the sensor 11a is supplied to a CPU 60. The CPU 60 controls a motor 9 through a motor controller 12, thus adjusting a rotational speed of the registration rollers 8. For example, the rotational speed of the registration rollers 8 is decreased to 1/2 of that in a sheet feeding/conveying mode. The transfer sheet P is decelerated to a low speed, and reaches a second position B. When the leading end of the transfer sheet is detected by a second sensor 11b, the transfer sheet is stopped under the control of the CPU 60 and the like. At this time, an image forming body 1 undergoes a charging operation by a charger 3, a write operation by a laser beam L, a developing operation by a developing unit 4, thus forming a toner image on the image forming body. The registration rollers 8 are rotated in synchronism with the image formation timing, and the transfer sheet P is conveyed to a transfer region in correspondence with a peripheral velocity of the image forming body 1. Thus, the toner image formed on the image forming body 1 is transferred onto the transfer sheet P by a transfer electrode 5. After the transfer operation, residual toner on the image forming body 1 is removed by a cleaning device 2.
FIG. 2 is a timing chart showing feeding and conveying timings of the transfer sheet P in the image forming apparatus shown in FIG. 1. A VSREQ signal represents an image output permission signal, a VSYNC signal represents an image output sync signal, a VIDEO signal represents an image signal, T.sub.O represents a period after the image output sync signal is output until the image signal is output, and T.sub.R represents a period after the image output sync signal is output until the registration rollers 8 are driven. As can be seen from this chart, the transfer sheet P is conveyed to the first position A at high speed by the pickup roller 7 and the registration rollers 8. When the leading end of the transfer sheet is detected by the first sensor 11a, the high-speed conveying mode is switched to a low-speed conveying mode at a speed about 1/4 of that in the high-speed conveying mode, and the transfer sheet is conveyed to the position B. When the leading end of the transfer sheet is detected by the second sensor 11b, the transfer sheet is stopped.
The registration rollers 8 are rotated again in synchronism with the VSYNC signal, so that the transfer sheet P is conveyed to the transfer region.
According to Japanese Patent Laid-Open (Kokai) No. 62-208065, as shown in FIG. 1, the transfer sheet P is satisfactorily caught by the registration rollers 8, and is then conveyed to the second position B at low speed. As a result, a slip of the transfer sheet P and a variation in the leading end position of the transfer sheet at the second position B can be eliminated.
However, in the prior art technique, a conveying speed must be immediately decreased to about 1/4 during a conveying operation by the registration rollers 8. Such an operation causes a speed change error in a conventional speed change device, and the low-speed conveying operation between the first and second positions A and B becomes very unstable. When the speed change operation is abruptly performed, the speed change device may be damaged. Since a plurality of sensors are used after the registration rollers 8, this makes the apparatus complex, and a traveling distance to the transfer region is prolonged. As a result, the apparatus becomes bulky, thus increasing cost.
In order to precisely convey a transfer sheet in synchronism with the image formation timing, the transfer sheet must be subjected to necessary countermeasures for preventing, e.g., double feeding or skew before it reaches the registration rollers 8. Since the prior art does not take such countermeasures, satisfactory precision cannot be assured although conveying control of the transfer sheet P is performed using a plurality of sensors after the registration rollers 8.
The problem of misregistration of the leading end position of the transfer sheet is particularly important in a color image forming apparatus. In this apparatus, an original is scanned to obtain color recording signals of yellow Y, magenta M, cyan Cy, black Bk, and the like, and the recording signals are written on an image forming body through a laser beam in units of colors to form an electrostatic latent image. The latent image is developed by a developing agent including a color toner to form a color toner image, and the color toner image is transferred onto a transfer sheet. This process is repeated. The transfer sheet is then separated from a transfer drum (image forming body) and is subjected to fixing to form a color image.